In an electronic circuit, due to the presence of fluctuation in the industrial power network, for example, when other electrical appliances are connected to or disconnected from the power grid, and due to incorrect wiring or device failure, when a power circuit is turned on, instantaneous charging of a capacitive element and other factors may lead to high current surge, to cause electronic devices such as LEDs to be damaged. In contrast, the other situation is that grid voltage fluctuations result in that under-voltage occurs instantaneously in the circuit, causing the working current to be lower than the normal working current value.
In order to solve the problem, it is necessary to use an electric heating element as a current limiter to protect electronic elements, through the parameter performance of the electric heating element, for example, the change of the resistance, it is possible to increase the resistance and decrease the current in the case of high current, it is possible to decrease the resistance to increase the current in the case of insufficient current, and in the case of stable current, the resistance can be restored to the normal resistance value to achieve a current limiting effect, to protect elements in the circuit. Generally, the electric heating element used is divided into two types, one type is an electric heating element with a positive temperature coefficient of resistance, when the current is increased, the electric heating element emits heat, and the temperature rises to cause the resistance to increase rapidly, thereby decreasing amplitude of the current. Moreover, for the circuit where the capacitive element is present, for example, an AC-DC power circuit, at startup, the capacitor is charged to lead to an instantaneous peak current, and the other type of electric heating element with a negative temperature coefficient of resistance is used, that is, in an initial stage, the electric heating element has higher resistance, for limiting instantaneous surge current of capacitor charging in the beginning, and when the capacitor works normally, the resistor starts emitting heat and the temperature rises, resulting in that the resistance is decreased and consumption is reduced.
However, the electric heating element, when used for limiting the current, has the following disadvantages and shortcomings:
1) The change of the resistance value of the electric heating element depends on a temperature change caused by heat conduction, rise and fall of the temperature generally require a certain time, therefore, the response speed is relatively slow and the change of the temperature depends on an environment where the electric heating element is, for example, a change of an environment temperature, and other elements possibly existing in the vicinity of the electric heating element, for example, a PCB board and the like, may affect the corresponding quantity and the corresponding speed thereof;
2) if the current in the circuit continuously generates peak values at an interval of a very short time, the electric heating element can change correspondingly in the beginning, and for the subsequent change, since the temperature cannot be restored to a stable working state within a short time, the electric heating element cannot play a role of limiting the current; and
3) the electric heating element can only limit the surge current in the circuit but cannot play a role in a situation where the current is lower than the normal value, to cause the amplitude of decrease of the current to be limited.